CHAPTER IX. 

 THE INTEGUMENT. 



The covering of the vertebrate body, the integument, is 

 composed of the skin and the various structures, such as scales, 

 claws, feathers, nails, hair, and glands. The integument is in 

 direct contact with the environment and its modifications give 

 protection from injury; prevent loss of body fluids; receive such 

 stimuli as those of touch, pain, or temperature, leading to adjust- 

 ment to external conditions; aid in the elimination of certain wastes; 

 and resist the entrance of injurious substances. 



The skin of all vertebrates is divisible into two layers, an outer 

 layer, the epidermis, composed of stratified epithelium, and an 

 underlying layer of connective tissue, the dermis or corium. 



Epidermis. — The epidermis is derived from embryonic ectoderm 

 and is divisil^le into two regions. The layer of cells which rests on 

 the corium is called the stratum germinativum, or Malpighian 

 layer. Cells of this layer are in contact with the nutritive fluids 

 carried m the coriiun, and by growth and di\'ision they gi\'e rise to 

 new cells that are gradually pushed outward to form the superficial 

 second layer, the stratum corneum. As the outermost cells of the 

 stratum corneum are worn away, other cells are added from the 

 stratum germinativum. Invaginations of the stratum germina- 

 tivum give rise to certain glands which are associated with the 

 skin. Both the stratum germinativum and the stratum corneum 

 are in\'olved in the formation of such structin-al modifications of 

 the integument as hair, claws, feathers, and nails. The composi- 

 tion of the epidermis \'aries in difl'erent animals and in dift'erent 

 regions of the same individual. 



Dermis (Corium). — The dermis, or corium, is derived from meso- 

 derm and is composed mainly of fibrous connective tissue, with 

 some intermingling elastic fibers supporting blood vessels and lym- 

 phatics. Smooth muscle cells may also be present in the region of 

 connective tissue. Chromatophores, too, are generally (Hstributed 

 in this region. Sense organs and nerve fibers are scattered through- 

 out the corium, and free nerve endings extend into the l)asal layers 

 of the epidermis. 



